[binutils-gdb] [gdb/ada] Fix gdb.ada/dynamic-iface.exp with gcc 7

[Tom de Vries <vries@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=05527d8ca1082b4607e9ddc3209691f454b3b186

commit 05527d8ca1082b4607e9ddc3209691f454b3b186
Author: Tom de Vries <tdevries@suse.de>
Date:   Mon May 23 14:50:02 2022 +0200

    [gdb/ada] Fix gdb.ada/dynamic-iface.exp with gcc 7
    
    This test in test-case gdb.ada/dynamic-iface.exp passes with gcc 8:
    ...
    (gdb) print obj^M
    $1 = (n => 3, a => "ABC", value => 93)^M
    (gdb) PASS: gdb.ada/dynamic-iface.exp: print local as interface
    ...
    but fails with gcc 7:
    ...
    (gdb) print obj^M
    $1 = ()^M
    (gdb) FAIL: gdb.ada/dynamic-iface.exp: print local as interface
    ...
    
    More concretely, we have trouble finding the type of obj.  With gcc 8:
    ...
    $ gdb -q -batch main -ex "b concrete.adb:20" -ex run -ex "ptype obj"
      ...
    type = <ref> new concrete.intermediate with record
        value: integer;
    end record
    ...
    and with gcc 7:
    ...
    type = <ref> tagged record null; end record
    ...
    
    The translation from tagged type to "full view" type happens in
    ada_tag_value_at_base_address, where we hit this code:
    ...
      /* Storage_Offset'Last is used to indicate that a dynamic offset to
         top is used.  In this situation the offset is stored just after
         the tag, in the object itself.  */
      if (offset_to_top == last)
        {
          struct value *tem = value_addr (tag);
          tem = value_ptradd (tem, 1);
          tem = value_cast (ptr_type, tem);
          offset_to_top = value_as_long (value_ind (tem));
        }
    ...
    resulting in an offset_to_top for gcc 8:
    ...
    (gdb) p offset_to_top
    $1 = -16
    ...
    and for gcc 7:
    ...
    (gdb) p offset_to_top
    $1 = 16
    ...
    
    The difference is expected, it bisects to gcc commit d0567dc0dbf ("[multiple
    changes]") which mentions this change.
    
    There's some code right after the code quoted above that deals with this
    change:
    ...
      else if (offset_to_top > 0)
        {
          /* OFFSET_TO_TOP used to be a positive value to be subtracted
             from the base address.  This was however incompatible with
             C++ dispatch table: C++ uses a *negative* value to *add*
             to the base address.  Ada's convention has therefore been
             changed in GNAT 19.0w 20171023: since then, C++ and Ada
             use the same convention.  Here, we support both cases by
             checking the sign of OFFSET_TO_TOP.  */
          offset_to_top = -offset_to_top;
        }
    ...
    but it's not activated because of the 'else'.
    
    Fix this by removing the 'else'.
    
    Tested on x86_64-linux, with gcc 7.5.0.
    
    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29057

Diff:
---
 gdb/ada-lang.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/gdb/ada-lang.c b/gdb/ada-lang.c
index 7e4988be5d0..6ab01fd27d4 100644
--- a/gdb/ada-lang.c
+++ b/gdb/ada-lang.c
@@ -6492,7 +6492,8 @@ ada_tag_value_at_base_address (struct value *obj)
       tem = value_cast (ptr_type, tem);
       offset_to_top = value_as_long (value_ind (tem));
     }
-  else if (offset_to_top > 0)
+
+  if (offset_to_top > 0)
     {
       /* OFFSET_TO_TOP used to be a positive value to be subtracted
 	 from the base address.  This was however incompatible with